1. Field of the Invention
The present invention relates to phase-array antennas and, more particularly, to millimeter (mm) wave, electronically scannable antennas.
2. Description of the Prior Art
A phase-array antenna is an antenna with two or more driven elements. The elements are fed with a certain relative phase, and they are spaced at a certain distance, resulting in a directivity pattern that exhibits gain in some directions and little or no radiation in other directions.
Phased arrays can be very simple, consisting of only two elements. For example, a simple phased array may be formed from two dipoles spaced a quarter wavelength apart in free space. If the dipoles are fed 90 degrees out of phase, radiation from the two dipoles will add in phase in one direction and cancel in the opposite direction. In this case, the radiation pattern is unidirectional having one major lobe. Phased arrays can have directivity patterns with two, three or more different optimum directions. A bidirectional pattern can be obtained, for example, by spacing the dipoles at one wavelength, and feeding them in phase.
More complicated phased arrays are used by radio transmitting stations. Several vertical radiators, arranged in a specified pattern and fed with signals of specified phase, produce a designated directional pattern. This is done to avoid interference with other broadcast stations on the same channel.
Phased arrays can have rotatable or steerable patterns as well as fixed directional patterns. For example, an array of antenna elements may be mounted on a rotator that physically moves the array, usually periodically, such that its major lobe scans over all points in a given space. Alternatively, the major lobe may be moved electronically by varying the relative phase which will cause the directional pattern to be adjusted.
The use of slotted antenna arrays for forming directional mm wave antennas is also well known. Slotted antenna arrays for the reception of television signals from satellite transmitters are described by Collier in "Microstrip Antenna Array for 12 GHz TV", Microwave Journal, vol. 20, no. 9, pp 67, 68, 70, 71, Sept. 1977. The Collier antennas include arrays of 2, 4, 16, 64 and 512 radiating slots formed in a conductive sheet with slot spacings of a wavelength in the H-plane and half a wavelength in the E-plane. The energy distribution feeder for each array is a strip-line branching network that forms a microstrip with the slotted conductive sheet.
A slotted array antenna designed for maximum directivity is described in "mm-Wave Oversized Cavity Slotted Array", Microwave Journal, July 1984, pp. 147-149, by Klaus Salbach. The Salbach antenna is a two-dimensional array of slotted cavities using a broad hollow waveguide that is excited by a line-source array in the form of a conventional slotted waveguide with phase reversal of the slots in order to excite the desired mode.
Electronically scannable, phase-array antennas have found wide use in radar systems such as those required for surveillance, obstacle avoidance and target acquisition. Such antennas are usually massive structures that require complex networks to properly feed the antenna elements. Although they are complex and expensive, phase-array radars are used widely because of their reliability. For example, a phase-array radar has a gradual failure mode and will continue to function even if a number of individual antenna elements fail.
Those concerned with the development of electronically scannable, phase-array antennas have long recognized the need for reducing their size, complexity and cost. The present invention fulfills this need.